This invention relates generally to the field of digitally controlled printing devices, and in particular to liquid ink drop-on-demand printheads which integrate multiple nozzles on a single substrate and in which a poised liquid meniscus on a nozzle is expanded to a pre-determined volume and is separated for printing by thermal activation. Furthermore, the volume separated drop may be controlled to permit continuous toning and grayscale toning of images.
Ink jet printing has become a preferred technology for the printing of color images. The term xe2x80x9cink jetxe2x80x9d as used herein is intended to include all drop-on-demand or continuous ink jet propulsion systems including, but not limited to, bubble jet, thermal ink jet, piezoelectric and continuous.
There are two types of drop-on-demand ink jet printers that dominate the market. Drop-on-demand thermal ink jet printers operate by rapidly heating a small volume of ink, causing it to vaporize and expand, thereby ejecting ink through an orifice or nozzle and causing it to land on selected areas of a recording medium. The sequenced operation of an array of such orifices moving past a recording medium writes a dot pattern of ink on the recording medium, forming text or pictorial images. The print head typically includes an ink reservoir and channels to replenish the ink to the region in which vaporization occurs. An arrangement of thermal ink jet heaters, ink channels, and nozzles is disclosed in U.S. Pat. No. 4,882,595.
The drop-on-demand piezoelectric printers operate by using a separate piezoelectric transducer for each nozzle generating a pressure pulse to expel the drops. U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970, discloses a drop-on-demand ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Color rendition is accomplished by adding a few (typically three) color ink reservoirs and associated nozzles and ejection means so that dots of different colors may be overlaid on an appropriate recording medium.
The above methods as practiced suffer from drawbacks, notable the difficulty of achieving continuous tone (grayscale) color reproduction. One method used to provide continuous tone color reproduction, namely the deposition of multiple drops with the same volume onto a single image pixel, allows only a limited number of gray levels corresponding to the number of drops deposited. The volume of ink drops has also been controlled in piezoelectric drop-on-demand printers by varying the applied energy, such as by adjusting the pulse height or pulse width of the applied electrical signal. This method tends to allow only a small volume variation and exhibits a drop velocity variation with size, making placement difficult on the recording medium.
Commonly assigned co-pending U.S. patent applications Ser. No. 08/750,438 entitled A LIQUID INK PRINTING APPARATUS AND SYSTEM filed in the name of Kia Silverbrook on Dec. 3, 1996, discloses a liquid printing system that affords significant improvements toward overcoming the prior art problems associated with power usage, durability, thermal stresses, other printer performance characteristics, manufacturability, and characteristics of useful inks. The invention provides a drop-on-demand printing mechanism wherein the means of selecting drops to be printed is by thermal reduction of the surface tension of the selected drop producing a difference in position between selected drops and drops which are not selected but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an additional means is provided to cause separation of the selected drops from the body of ink. To cause separation of the drop the system requires either proximity mode, for which the recording medium must be in close proximity to the orifice in order to separate the drop from the orifice, or the use of an electric field between recording medium and orifice. A drop volume adjustment can be made, for example in proximity mode, by altering the distance between print head and recording medium which requires increased system complexity.
Commonly assigned co-pending U.S. patent application Ser. No. 08/787,657 entitled INK PRINTING WITH DROP SEPARATION filed in the names of James M. Chwalek and John A. Lebens on Jan. 21, 1997, discloses a printing system where by the separation of the selected drop is achieved by controlled thermal activation propelling it to the recording medium without the need for proximity or an electric field.
It is an object of the present invention to improve upon the above invention by demonstrating a new mode of operation. This mode, which was not previously predicted, permits control of the separated drop volume for continuous toning and gray scale toning of images. A volume change of at least a factor of 3 can be obtained with only a small change in drop velocity.
It is another object of the present invention to demonstrate a new mode of operation for a drop-on-demand printhead wherein electrothermal pulses applied to an annular heater located around the rim of a nozzle controls the expansion of a poised meniscus, the separation of the drop, and the volume of the separated drop, propelling it to the recording medium. Electrothermal pulses applied to selected nozzles heat the ink in those nozzles, altering material properties of the ink, including a reduction in the surface tension of the ink and causing it to expand past its initially poised state. Heating the ink adjacent to the heater surface to a temperature greater than its boiling point results in separation of the drop. By controlling the heating, a pre-determined drop volume may be delivered to the recording medium. This pre-determined drop volume may consist of more than one drop ejected as a result of a singly-applied electrothermal pulse.
According to a feature of the present invention, an ink drop ejecting printhead includes a substrate having an ink drop emitting opening; a heater on the substrate surrounding the opening; and an ink supply communicating with the opening to supply ink, whose surface tension decreases inversely with its temperature, to the opening under positive pressure relative to ambient. A variable-energy electrical power supply connected to the heater, whereby application of an electrical pulse of sufficient energy to the heater will cause separation of an associated ink drop from the ink supply. A power supply control is adapted to regulate the energy of electrical pulses applied to the heater from the power supply, whereby the volumes of separated ink drops are proportional to the energy of the associated electrical pulses.
According to another feature of the present invention, a process for ejecting ink from a printhead includes the steps of communicating an ink supply, whose surface tension decreases inversely with its temperature, with an ink-emitting opening to supply ink to the opening; applying positive pressure relative to ambient to the ink supply; adjustably applying pulses of heat to the ink at the opening of sufficient energy to cause separation of associated ink drops from the ink supply; and variably adjusting the applied heat pulse energy, whereby the volume of the separated ink drops are proportional to the energy of associated heat pulses.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.